1. Field of the Invention
The invention relates to a lithium secondary battery, and more particularly to a lithium secondary battery which can be applied to a vehicle-mounted power source, and a manufacturing method of the same.
2. Description of Related Art
In a lithium secondary battery, a portion of a non-aqueous electrolyte is decomposed during initial charging such that a protective film (solid electrolyte interface film (SEI film)) containing the decomposed matter is formed on the surface of a negative electrode. The interface between the negative electrode and the non-aqueous electrolyte is stabilized by the SEI film and thus the durability (for example, cycle characteristics) of the battery can be enhanced. As documents in the related art, there are Japanese Patent Application Publication No. 2011-187440 (JP 2011-187440 A) and Japanese Patent Application Publication No. 2009-026691 (JP 2009-026691 A). For example, in JP 2011-187440 A, it is described that a fluoro salt as an additive is included in a non-aqueous electrolyte to form an SEI film containing components derived from the fluoro salt on the surface of a negative electrode, and thus the durability of a battery can be enhanced.
However, according to the investigations by the inventors, in a case where the above-described technique is applied to a battery (for example, in-vehicle battery) that requires high input and output densities and a high capacity retention ratio over a long period of time, for example, over 10 years, there is still room for improvement. That is, the SEI film may deteriorate or peel off from the surface of the negative electrode while the battery is continuously used, and thus it is difficult to maintain a stable SEI film over a long period of time. In addition, at a position at which the SEI film is deteriorated, a new SEI film is formed. At this time, typically, the non-aqueous electrolyte is consumed, and this may result in a reduction in battery capacity. Therefore, as the use period of the battery increases, there is a problem in that the resistance increases or the capacity retention ratio decreases. On the other hand, when designs for suppressing an increase in the resistance are employed, initial characteristics (for example, initial resistance) may be degraded. That is, it is difficult to make the initial characteristics and the durability of the battery be compatible with each other.